Subcritical fluid chromatography

Achiral-chiral chromatography has also been accomplished using subcritical fluid chromatography (Phinney et al., 1998). In this work, the structurally related [3-blockers, 1,4-benzodiazepines, and two cold medicines were separated using methanol or ethanol modified carbon dioxide mobile phases. The (3-blockers were separated using cyanopropyl and Chiracel OD columns connected in series. Likewise, an amino bonded phase and Chiracel OD column were used for the separation of the 1,4-benzodiazepines. Guaifenesin and phenylpropanolamine from cough syrup were separated on cyanopropyl and Chiralpak AD columns in series. 

Sandra, P, Kot, A., David, F. (1994). Subcritical fluid chromatography of coupled CSP columns for the separation of drug enantiomers. Chim. Oggi. 12, 33-36. 

A screening approach was described in subcritical fluid chromatography for basic compounds by Stringham [52]. The analyzing conditions of the screening step are the following a flowrate of 2 mL/min, 180 bars of backpressure, and room temperature. The column for screening was Chiralpak AD-H, i.e. 5-p.m material was used. The mobile phase contained CO2, 20% of ethanol, and 0.1 % ethanesulfonic acid (ESA)... 

Stringham, R.W Chiral separation of amines in subcritical fluid chromatography using polysaccharide stationary phases and acidic additives. J. Chromatogr. A 2005, 1070, 163-170. 

Figure 7.4 The Subcritical Fluid Chromatography range. This occupies the volume in the phase diagram below the locus of critical temperatures, above and below the locus of critical pressures, and is composed mostly of the more volatile mobile-phase component. Reproduced by permission of the American Chemical Society.

C. Wolf and W. H. Pirkle, Enantioseparations by subcritical fluid chromatography at cryogenic temperatures , J. Chromatogr. 785 173-178 (1997). 

Peytavin et al. [17] have reported on the chiral resolution of mefloquine, halofantrine, enpiroline, quinine, quinidine, chloroquine, and primaquine by subcritical fluid chromatography on a (S) naphthylurea column (250 X 4.6 mm ID). The mobile phase consisted of carbon dioxide, methanol, and triethylamine at a 3-ml/min flow rate. Except for primaquine and... 

T. Loughlin, R. Thompson, G. Bicker, R Tway, and N. Grinberg, Use of subcritical fluid chromatography for the separation of enantiomers using packed column cellulose based stationary phase. Chirality 8 (1996), 157. 

Subcritical fluid chromatography using a carbon dioxide/methanol mobile phase has been performed on diastereo-meric mixtures of isoxazolines 28-30. 

Normal phases composed of hexane modified with 2-propanol have been successfully used with the 3,5-dimethylphenyl carbamate and para-toluoyl ester forms of the CD CSP. The P-CD CSP has been used in subcritical fluid chromatography with a mobile phase composed of carbon dioxide modified with methanol, ethanol, or 2-propanol (69). Under these conditions, racemic amides and phosphine oxides were enantiomerically resolved. 

Terfloth, G. Enantioseparations in super-and subcritical fluid chromatography,... 

Liu,Y., Berthod, A., Mitchell, C. R., Xiao,T. L., Zhang, B., Armstrong, D. W. Super/subcritical fluid chromatography chiral separations with macrocyclic glycopeptide stationary phases, J. Chromati. A, 2002, 978, 185-204. 

Camel V, Thiebaut D, Caude M, Dreux M. Packed column subcritical fluid chromatography of underivatized amino acids. J Chromatogr 1992 605 95-101. Anton K, Bach M, Geiser A. Supercritical fluid chromatography in the routine stability control of antipruritic preparations. J Chromatogr 1991 553 71-79. Giddings JC, Meyers M, Wahrhaftig AL. Int J Mass Spectrom Ion Physi 1970 4 9-20. 

Blum AM, Lynam KG, Nicolas EC. Use of a new Pirkle-type chiral stationary phase in analytical and preparative subcritical fluid chromatography of pharmaceutical compounds. Chirality 1994 6 302-313. 

Liu Y, Berthod A, Mitchell CR, Xiao TL, Zhang B, Armstrong DW. Super/ subcritical fluid chromatography chiral separations with macrocyclic glycopep-tide stationary phases. J Chromatogr A 2002 978 185-204. 

Ibanez, E., Lopez-Sebastian, S., Tabera, J., and Reglero, G. 1998. Separation of carotenoids by subcritical fluid chromatography with coated, packed capillary columns and neat carbon dioxide. 

This section covers three instrumental techniques gas-chromatography (GC), high-performance liquid chromatography (HPLC) and super/subcritical fluid chromatography (SFC). One additional chromatographic technique which is not intensively used for enantiosepara-tions at present is thin-layer chromatography (TLC). 

Super- and subcritical fluid chromatography (SFC) underwent extremely rapid development in the mid-1980s, and was believed to be a technique with a great potential that combined the advantages of GC and HPLC. However, inherent fundamental and technical limitations of this technique became apparent and led to some stagnation in its further development. 

A. M. Blum and K. G. Lynam, Use of a New Pirkle-Type Chiral Stationary Phase in Analytical and Preparative Subcritical Fluid Chromatography of Pharmaceutical Compounds, Chirality, 6(1994)302. 

P. Macaudiere, M. Caude, R. Rosset and A. Tambute, Resolution or Racemic Amides and Phosphine Oxides on a P-Cyclodextrin-Bonded Stationary Phase by Subcritical Fluid Chromatography, J. Chromatogr., 405(1987)135. 

Subcritical fluid chromatography (SubFC) analysis of nine monosaccharides and three polyols, meso-erythritol, xylitol, and mannitol, on silica phases has been developed. Mobile phase composition was CO2 modifier (80 20, v/v), the modifier being MeOH-water-triethylamine (91.5 8.0 0.5, v/v/v). In SubFC, the addition of triethylamine slightly increase the retention since interactions with the solutes and/or a strong interaction with the silanols and the residual silanols can occur. 

The general confusion about the physical and chemical meaning of the words super and critical has lead to a plethora of terms such as subcritical fluid chromatography, near-critical fluid chromatography, and enhanced fluid chromatography to deal with the fluids when they are nearly but not quite super critical. All these names, including SFC, actually describe the same technique. 

The main virtue of SFC compared with HPLC is that it provides enhanced diffusivity. Enhanced diffusivity can also be obtained, however, in other ways. First, CO2 in its liquid state provides higher diffusivity than do conventional liquids application of this mobile phase is often called subcritical fluid chromatography. Another possibility to improve diffusivity is to use mixtures of liquid CO2 and conventional liquids as the mobile phase. This approach has been taken by Olesik and co-workers (Cui and Olesik, 1991 Lee and Olesik, 1995 Lee, Olesik and Fields, 1995) who showed that the mixing of methanol with up to 50% liquid CO2 resulted in increased diffusivity and decreased viscosity and the polarity was largely maintained. A third approach to HPLC with enhanced diffusivity is the application of enhanced separation temperatures as demonstrated by Trones, Iveland and Greibrokk (1995). 

Phinney, K.W. Sander, L.C. Wise, S.A. Coupled achiral/ chiral column techniques in subcritical fluid chromatography for the separation of chiral and nonchiral compounds. 

When unified chromatography was first proposed, the chromatographic techniques were divided into GC, SFC, and LC. Nowadays, there has been a general acceptance that GC and LC are extreme mobile-phase conditions and that several intermediate conditions are possible. This includes solvated GC enhanced LC, and subcritical fluid chromatography, among other possibilities. Considering... 

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